Two papers produce models of the how and why of the geysers of water that are …

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One of the surprises that came as we started exploring the solar system was that a number of the moons we imaged were geologically active. Instead of being powered by internal heat, as the plate tectonics of Earth are, the moons appeared to be heated by gravity. In the case of Jupiter's moon Io, the planet's mass and that of the outer moons combined to push and pull Io's interior, creating friction, heat, and spectacular volcanoes.

Saturn's moon Enceladus provided a similar surprise to Cassini, which imaged plumes of water erupting from its surface, such as the one seen above. But Enceladus is much further from the sun, and interacts with fewer of Saturn's moons, so it was difficult to pin down why it was so active. Two papers in this week's Nature propose some models for the how and why of these watery plumes.

First, the how: what powers the plumes? The authors of this paper suggest that the surface of Enceladus is covered with large blocks of ice, somewhat like the tectonic plates that cover the Earth's surface. These are thick but float on top of a deep reservoir of water, and so can flex independent from the moon's core. And the orbital dynamics of Enceladus ensure that they do.

On average, Enceladus keeps a single surface pointed towards Saturn because its orbit and rotation take an equal amount of time. But its orbit is eccentric; as a result, the moon moves more quickly when close to Saturn and more slowly when further away. The net result is that the point closest to Saturn changes throughout the orbit, putting stress on the moon's surface and causing the joints between the icy blocks there to flex.

Why does this flexing produce the geysers? That's where the second paper comes in. The authors of this paper suggest that the plumes don't actually result from the deep reservoir of water making their way to the moon's surface. Instead, they suggest that the friction of the blocks of ice grinding past each other is sufficient to melt some of the ice, which then explodes into space. They calculate that a 500 km fault is sufficient to generate nearly a gigawatt of power.

Generally, testing models about what goes on outside of our planet can be pretty challenging, but the stress models in the first paper are detailed enough that they predicted specific faults that would be active the next time Cassini did a close flyby, which was only three weeks ago. No word yet on whether their predictions were borne out.